1. Field of the Invention
The invention relates to shaped articles formed from glass fiber reinforced cementitious compositions, especially glass fiber reinforced gypsum, but more particularly to a process and apparatus for making such shaped articles.
2. Description of the Prior Art
Gypsum has been used as a casting and molding material for many years. Gypsum is known as plaster of Paris and is the hemi-hydrate of calcium sulfate. 100 parts by weight of the gypsum hemi-hydrate combine stoichiometrically with 18.6 parts by weight of water to form a hard, set plaster containing two mols of combined water. In order to prepare a workable, pumpable, moldable, dough-like composition, the gypsum hemi-hydrate is combined with an excess of water in addition to the 18.6 parts by weight which are required for the conversion of the hemi-hydrate into a set plaster. With ordinalry calcined gypsum, also known as beta hemi-hydrate, the gypsum customarily is combined with more than 50 percent of its weight of water in order to achieve a pouring consistency. It is possible to achieve a pouring consistency with less than 50 percent water when the gypsum is in the form of crystaline calcined gypsum also known as alpha hemi-hydrate. See U.S. Pat. No. 1,901,051--RANDEL et al. Moldable compositions containing 40 parts of water for every 100 parts of dry powder (predominantly alpha hemi-hydrate) have been described. See U.S. Pat. No. 2,494,403--NIES et al.
Structurally reinforced articles formed from gypsum and glass fibers have been described wherein the glass fibers are mixed into a slurry of gypsum hemi-hydrate and water. See U.S. Pat. Nos. 3,062,670--MARZOCCHI et al; 2,681,863--CROCE et al and 3,147,127--SHANNON. In all of these glass fiber reinforcement processes involving preparation of a slurry containing glass fibers, the act of mixing the fibers introduces a tendency to break the fibers into short lengths. It has been reported that 0.1 percent of textile fibers (diameter 0.0004 inch) cannot be admixed with the gypsum slurry whereas 3 percent of larger diameter fibers (0.003 inch) can be added to a moldable gypsum slurry with ease. See U.S. Pat. No. 3,062,670 supra.
A significant development in glass fiber reinforced gypsum technology is set forth in British patent 1,204,541--National Research Development Corporation. The significant new development avoids mixing of glass fibers in a gypsum slurry but instead prepares an admixture of gypsum hemihydrate, water and glass fibers by spraying an aqueous slurry of the gypsum hemi-hydrate into a stream of freshly chopped glass fibers or onto a preformed mat formed from randomly oriented glass fibers. Glass fibers reinforced gypsum is known as GRG.
In order to achieve adequate wetting of the glass fibers in the British process, a substantial excess of water is employed in the aqueous slurry--that is, an excess over the stoichiometric amount required to combine with the gypsum hemi-hydrate. Slurries containing 50 percent by weight of water and more are proposed. The British patent process thus prpepares a watery slurry containing gypsum hemi-hydrate and glass fibers. The excess water is initially removed by vacuum removal or by pressure to produce a composition which still contains an excess of water over the stoichiometric amount required for the gypsum hemi-hydrate and contains enough water to provide a moldable and workable plaster which exists for a short period of time until the gypsum becomes set. The removal of the excess water is a difficult task. One technique for removing the water has been to form the dilute slurry on a porous formable sheet, such as a sheet of Kraft paper, and to pass the porous formable sheet containing the dilute slurry over a suction box which has facilities for extracting water from the dilute slurry through the Kraft paper which functions as a filter septum. Nonetheless, the British patent process is capable of producing glass fiber reinforced gypsum articles of remarkable strength characteristics as a result of retaining relatively long length glass fibers in a random orientation in the final article.
It would certainly be desirable to eliminate the cumbersome and expensive water removal stage which is necessitated in the process described in the British patent. It is also desirable to develop a process for producing glass fiber reinforced gypsum articles on a continuous basis in a variety of profiled shapes. Such profiled shapes can be employed in producing products of the type described in co-pending U.S. Patent applications 293,331, filed Sept. 29, 1972; 328,968, filed Feb. 2, 1973; and 328,969 filed Feb. 2, 1973, which are assigned to an assignee of the present invention. The profiled shapes also can be employed to produce liner sheets for building construction panels as will be hereinafter described.
The process and apparatus which have been developed to produce articles from glass fiber reinforced gypsum are also applicable to treatment of other dough-like materials which can be shaped, then hardened, e.g., glass fiber reinforced cement (U.S. Pat. Nos. 3,716,386; 3,783,092).